1. Field of the Invention
The present invention relates to the treatment of gaseous effluents low in sulfur dioxide content to provide solutions of ammonium or an alkali or alkaline earth metal bisulfite, and, more especially, to the treatment of gaseous effluents low in sulfur dioxide content to convert same into sodium bisulfite solutions.
2. Description of the Prior Art:
It is known to this art that certain industrial gaseous effluents, in particular those emanating from sulfuric acid production units, and more particularly from plants for the catalytic oxidation of sulfur dioxide, typically referred to as tail gases, contain a certain amount of sulfur dioxide.
A number of processes have been developed to reduce the concentration of sulfur dioxide in these effluents, which also contain oxygen.
Thus, in the so-called double absorption process, the gaseous mixture following combustion is subjected to a first catalytic oxidation step and a first absorption step, and then the residual gas, which is less rich in sulfur dioxide, is again oxidized and transferred to a second absorption column. While this process ensures that the discharged effluent contains about 0.03% by volume of sulfur dioxide, it requires special apparatus with attendant very high investment costs.
Absorbing sulfur dioxide contained in the tail gases into a basic medium, for example caustic soda solutions, is also known to this art. However, the highly basic pH and the low concentration of sulfur in these solutions comprehends that it is very difficult to add value to the final solutions obtained on an industrial scale.
Absorption of sulfur dioxide in a slightly basic medium has also been proposed to this art. However, this absorption is generally accompanied by oxidation of sulfur (+IV) into sulfur (+VI), i.e., formation of sodium (or ammonium) sulfate due to the high oxygen concentration in the tail gases. This process thus presents the drawback of generating a saline effluent from a gaseous effluent.